1. Field of the Invention
This invention relates to a process for concentrating a solution of hydrogen peroxide and water and furnishing vapors of said concentrated solution to a sterilization chamber containing goods that require sterilant penetration to be sterilized.
2. Description of the Prior Art
U.S. Pat. Nos. 4,169,123 and 4,169,124 to Moore et al. and Forstrom et al., respectively, (hereinafter referred to collectively as "Moore and Forstrom") teach that gaseous hydrogen peroxide can be an effective sterilant and may be employed in a relatively "cold" sterilization process (that is, in the same general temperature range as that used, for example, in ethylene oxide sterilization processes) without the attendant disadvantages of known cold sterilization processes. Moore and Forstrom implement their gaseous hydrogen peroxide sterilization process by vaporizing in a closed sterilization chamber a relatively dilute liquid solution of hydrogen peroxide and water (in the range of 0.05 to 5% by weight hydrogen peroxide) and permitting the vapors so produced to contact items to be sterilized, which are located within the sterilization chamber, until sterilization is achieved.
Moore and Forstrom claim that sterilization may be effected by their process at remarkably low concentrations (on the order of 0.5 mg/liter) of hydrogen peroxide. Since hydrogen peroxide vapor, not water vapor, is the sterilizing agent in the Moore and Forstrom process, it may be seen that the effective concentration of sterilizing agent at the point of attack on the object to be sterilized would be relatively lower, due to the presence of water vapor, than a situation in which pure hydrogen peroxide vapor were present. That is, a given weight of hydrogen peroxide in liquid solution can produce only that weight of hydrogen peroxide in vapor form assuming complete vaporization. Clearly, therefore, it would be preferable to use highly concentrated hydrogen peroxide as the starting material in a gaseous hydrogen peroxide sterilization process. The handling requirements, however, for highly concentrated hydrogen peroxide solution (say above about 65% by weight) are so onerous because of the hazard posed by such a strong oxidant that the use of such highly concentrated solutions is impractical.
Moore and Forstrom are silent on the desirability of increasing the effective concentration of hydrogen peroxide vapor at the point of attack. They are also silent on a problem experienced in the application of their process when the nature of the goods to be sterilized requires that sterilant penetrate wrapping material and/or long, small-diameter tubes (lumens) such as are present on endoscopes. Indeed, this requirement for sterilant penetration is often present with heat-sensitive goods that may not be subjected to steam sterilization and thus require "cold" sterilization. The problem discovered with the Moore and Forstrom process where sterilant penetration is needed centers around the wellknown difference in volatility and sonic velocity between water and hydrogen peroxide; specifically, water is more volatile than hydrogen peroxide and vaporizes first; further water vapor has a higher translational and sonic velocity than hydrogen peroxide vapor and thus a higher diffusion rate in air than hydrogen peroxide vapor. If, for example, a solution of hydrogen peroxide and water is injected into an evacuated sterilization chamber, the more volatile, faster-traveling water vapor penetrates tubes and wraps; once at these locations, the water vapor forms a barrier to penetration of the less volatile, slower-traveling hydrogen peroxide vapor. Accordingly, the effective concentration of hydrogen peroxide vapor at the point of attack is diminished. Alternatively, if the liquid solution of hydrogen peroxide and water could be vaporized and the vapor homogenized prior to injection into the sterilization chamber, the procedure also would be unsuccessful from the standpoint of effective concentration of hydrogen peroxide vapor at the point of attack. First, water vapor dominates the vapor phase over a hydrogen peroxide-water solution such that, for example, an 18.5 mole % hydrogen peroxide liquid supports a vapor containing only 1.0 mole % hydrogen peroxide at room temperature. Such a relatively weak mixture of peroxide is an ineffective sterilant for narrow lumens. Second, the combination of the relative concentrations, sonic velocities and translational velocities of the two gases would tend to cause water vapor to penetrate narrow lumens ahead of the hydrogen peroxide vapor.
The foregoing establishes that the teachings of Moore and Forstrom fall far short of producing effective concentrations of hydrogen peroxide vapor at the point of attack where penetration of the vapor into wraps and narrow lumens is required. There exists, therefore, a need for a process that will enhance the concentration of hydrogen peroxide vapor at the point of attack with goods in the sterilizer that require the sterilant to traverse tortuous paths, such as wrapped goods, and penetrate narrow lumens.